The invention is directed to a novel photoresist composition that employs onium salt carboxylates. The photoresist composition combines either an onium carboxylate salt with a phenolic photoresist, such as novolac, or an onium cation protected carboxylate containing resin such as an acrylic/acrylic acid copolymer. The onium carboxylates provide a thermally stable dissolution inhibitor that, when exposed to light, promotes the dissolution of the photoresist, does not require water for the reaction, create nitrogen, and has acceptable radiation transmission characteristics.
There are a variety of applications for which thick film lithography can prove to be advantageous. For example, thick film lithography can be used to create plating molds to fashion small, high aspect ratio metal parts for micromechanical applications (e.g., LIGA), allow the semiconductor industry to extend the thickness of their bump technology, and provide for extension of thin film head manufacturers to extend their thickness regime. However, the photoresist compositions found to be generally useful for resist films thicker than 70 μm, such as SU-8 (MicroChem Corp., Marlboro, Mass.), have disadvantageous features; namely, solvent development and the highly crosslinked epoxy can be difficult to strip from a substrate. Consequently, there are good reasons to try to extend lithography as practiced by the semiconductor industry, i.e., thin film lithography, into the thick film regime.
Thin film lithography photoresists, such as positive working photoresists based on aqueous base soluble phenolic resins, have gained popularity because of their superior resolution potential, one micron with state-of-the art processes, and better etch resistance. Diazonapthoquinone/novolac (DNQ/novolac) is a positive working, multicomponent resist system consisting of a novolac resin prepared by the acid catalyzed co-polymerization of cresol and formaldehyde and a sensitizer, DNQ, which is base insoluble. Upon photolysis, DNQ produces a carbene which undergoes rearrangement to form a ketene. The ketene adds water, present in the resist film to form a base soluble indene carboxylic acid photo-product. DNQ/novolac resist systems are common in microcircuit manufacturing applications and consequently, there have been some attempts to extend the thickness and resolution capabilities of these materials to the thick film regime, film thicknesses of 75 μm or greater.
However, photoresists developed for thin film applications are not necessarily applicable to thicker film applications. For example, the DNQ/novolac resists discussed above suffer from several drawbacks. While DNQ/novolac photoresists are widely used, they are limited to film thickness below about 70 μm and the near UV (wavelengths greater than about 366 nm) because of the absorption characteristics of DNQ. Furthermore, DNQ/novolac resists form nitrogen gas upon exposure to radiation which causes bubbles in the resist film, and are generally not thermally stable enough to remove effectively the solvent from the thick films (generally greater than 75 μm) required for LIGA-type applications.
One approach to solving the problems associated with the use of thin film resist compositions for thick film applications is to replace DNQ in DNQ/novolac resists with a thermally stable dissolution inhibitor that when exposed to radiation promotes the dissolution of novolac but, at the same time, does not require water for the reaction. Moreover, it is desirable that the dissolution inhibitor chosen not create nitrogen or any other gaseous reactions products. The availability of such a dissolution inhibitor would allow the extension of novolac-based photoresists to thick film photoresist applications.